1. Field of the Invention
This invention is directed to a system and technique for remote infrared spectrum analysis and chemical analysis. The invention provides for the use of a solid state acousto-optic device or acousto-optic tunable filter (AOTF) in combination with fiber optic technology in order to provide a system for infrared spectrum analysis, chemical analysis and of the species of interest.
2. Description of Prior Art
The acousto-optic tunable filter (AOTF) is a birefringent crystal having an acoustic transducer bonded to one face. Broad band light radiation passing through a crystal can be diffracted into specific wavelengths by application of a radio-frequency driving signal to the crystal transducer. The AOTF device can be used as a part of a spectrometer or other optical instrument. The AOTF has several advantages over alternative spectroscopic techniques such as filter wheels, diffraction gratings, and Fourier transform infrared. The AOTF is small, fast, reliable and has no moving parts. Because of these features, the AOTF can be used for applications such as chemical process control, medical diagnostics, spectral radiometry, and real time composition analysis in the production environment. When coupled to fiber optics, the AOTF can be located remote from the sample of interest. This remote location has an advantage when the sample is in a harsh or potentially explosive environment. Fiber coupling reduces a chance of explosion by eliminating all electrical voltage in the sampling region. However, because of the low fiber optic throughput, and poor transmission at certain wavelengths, the sensitivity of the system is severely degraded when fiber optics are used. The sensitivity can also be reduced if the sample is highly absorbing, or if the AOTF has a very narrow optical bandpass.
U.S. Pat. No. 4,490,845, Steinbruegge et al. discloses an automated acousto-optic infrared analyzer system wherein an acousto-optical tunable filter (AOTF) is supplied with radio-frequency pulses typically about 3.5 microseconds long, 10 watts of peak power over an operating frequency range of 20-100 MHz to the transducer filter.
U.S. Pat. No. 4,490,845, Steinbruegge et al. discloses an acousto-optic tunable filter infrared analyzer consisting of a crystal and a variable frequency RF energy source whereby acoustic waves are launched in the crystal to interact with the selected narrow band-width portion of the polarized infrared radiation to make it distinguishable from the remaining radiation. The RF source provides an RF pulse which is of the proper width to provide an RF power duty cycle limited to a load level which does not overheat the crystal.
U.S. Pat. No. 4,663,961, Nelson et al. discloses an acousto-optical tunable filter for performing spectral analysis of chemicals at a remote location. This patent utilizes a pulse RF source having a duty cycle of 1% and a peak-to-peak voltage across the transducer of approximately thirty volts. The crystal which is disclosed to have been utilized in experiments is a noncollinear thallium arsenic selenide AOTF. The specification also discloses that the acoustical optical tunable filter can utilize a TeO.sub.2 crystal. In this disclosure, only a pulsed RF source is disclosed.
When pulsed RF sources are used, the signal to noise ratio is limited, the sensitivity of the system is lowered, and the throughput is lowered because of the low duty cycle.